Converted examples to use MacroGL entites

Converted the back ends examples to use MacroGL entites instead of raw OpenGL when possible and added an util to load several text files in one time (already possible to do with base text resource utility, but painful).
There is currently a bug in there that will actually associates the given texture unit (here GL.TEXTURE0), not to the given texture token, but to the previously binded texture token. But due to the semantic of glActiveTexture, I don't know how to solve this. Is there a way to have no active texture unit? I have the feeling there is always one selected (TEXTURE0 by default I think).